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POPULAR CONCHOLOGY. 



Family 4. — Placobranchiacea. Placobranchus 

 Hasselt ; Elysia Risso ; Chalidis Quatrefages ; Actasonina 

 Quatrefages ; Ictis Alder. ; Fucicola Quoy et Gaim. ; 

 Dermatobranclius Hasselt. 



Family 5. — Phyllidiacea. Phyllidia Cuvier ; Hy- 

 pobranchgea Adams ; Pleurophyllidia Stammer. 



Order VIII. 

 HETEROPODA. 



The foot of the animals of this order is changed into a 

 vertical fin, which serves for swimming ; the gills vary, 

 sometimes entirely absent ; the mouth is armed with great 

 horny hooks, for the seizing and tearing the prey ; some- 

 times the body is prolonged into a tail, which at the end 

 has a fin ; sometimes there are feelers, sometimes none ; 

 the same may be said with regard to eyes. The shell is 

 sometimes present, sometimes absent, and either covers 

 the whole or part of the body. — All marine. 



The Heteropoda are Molluscs which swim about freely 

 in the open sea, chiefly making their appearance at night, 

 or in the evening. They are limited to the warm and 

 temperate zones, and (especially in the former) cover the 

 sea in millions, and this for many nights in succession, so 

 that with a net vast numbers may be caught. Like the 

 Pteropoda they are always swimming about, and have 

 never been observed to fix themselves, but their motions 

 are much slower ; the movements of the Sagitta very 

 much resemble those of fish. Most Heteropoda are colour- 

 less, and transparent as water. What they feed on is not 



